Naturally occurring deposits containing oil and natural gas have been located throughout the world. Given the porous and permeable nature of the subterranean structure, it is possible to bore into the earth and set up a well where oil and natural gas are pumped out of the deposit. These wells are large, costly structures that are typically fixed at one location. As is often the case, a well may initially be very productive, with the oil and natural gas being pumpable with relative ease. As the oil or natural gas near the well bore is removed from the deposit, other oil and natural gas may flow to the area near the well bore so that it may be pumped as well. However, as a well ages, and sometimes merely as a consequence of the subterranean geology surrounding the well bore, the more remote oil and natural gas may have difficulty flowing to the well bore, thereby reducing the productivity of the well.
To address this problem and to increase the flow of oil and natural gas to the well bore, companies have employed the well-known technique of fracturing the subterranean area around the well to create more paths for the oil and natural gas to flow toward the well. As described in more detail in the literature, this fracturing is accomplished by hydraulically injecting a fluid at very high pressure into the area surrounding the well bore. This fluid must then be removed from the fracture to the extent possible to ensure that it does not impede the flow of oil or natural gas back to the well bore. Once the fluid is removed, the fractures have a tendency to collapse due to the high compaction pressures experienced at well-depths, which can be more than 20,000 feet. To prevent the fractures from closing, it is well-known to include a propping agent, also known as a proppant, in the fracturing fluid. The goal is to be able to remove as much of the injection fluid as possible while leaving the proppant behind to keep the fractures open.
Several properties affect the performance of a proppant. If forces in a fracture are too high for a given proppant, the proppant will crush and collapse, and then no longer have a sufficient permeability to allow the proper flow of oil or natural gas. In deep wells or wells whose formation forces are high, proppants must be capable of withstanding high compressive forces, often greater than 10,000 pounds per square inch (“psi”). Proppants able to withstand these forces (e.g., up to and greater than 10,000 psi) are referred to as high strength proppants. In shallower wells, high strength proppants may not be necessary as intermediate strength proppants may suffice. Intermediate strength proppants are typically used where the compressive forces are between 5,000 and 10,000 psi. Still other proppants can be used when the compressive forces are low. For example, sand is often used as a proppant at low compressive forces.
In addition to the strength of the proppant, how the proppant will pack with other proppant particles and the surrounding geological features is critical, as the nature of the packing can impact the flow of the oil and natural gas through the fractures. For example, if the proppant particles become too tightly packed, they may actually inhibit the flow of the oil or natural gas rather than increase it.
The nature of the packing also has an effect on the overall turbulence generated through the fractures. Too much turbulence can increase the flowback of the proppant particles from the fractures toward the well bore. This may undesirably decrease the flow of oil and natural gas, contaminate the well, cause abrasion to the equipment in the well, and increase the production cost as the proppants that flow back toward the well must be removed from the oil and gas.
The useful life of the well may also be shortened if the proppant particles break down. For this reason, proppants have conventionally been designed to minimize breaking. For example, U.S. Pat. No. 3,497,008 to Graham et al. discloses a preferred proppant composition of a hard glass that has decreased surface flaws to prevent failure at those flaws. It also discloses that the hard glass should have a good resistance to impact abrasion, which serves to prevent surface flaws from occurring in the first place. These features have conventionally been deemed necessary to avoid breaking, which creates undesirable fines within the fracture.
The shape of the proppant has a significant impact on how it packs with other proppant particles and the surrounding area. Thus, the shape of the proppant can significantly alter the permeability and conductivity of a proppant pack in a fracture. Different shapes of the same material offer different strengths and resistance to closure stress. It is desirable to engineer the shape of the proppant to provide high strength and a packing tendency that will increase the flow of oil or natural gas. The optimum shape may differ for different depths, closure stresses, geologies of the surrounding earth, and materials to be extracted.
The conventional wisdom in the industry is that spherical pellets of relatively uniform size are the most effective proppant body shape to maximize the permeability of the fracture. See, e.g., U.S. Pat. No. 6,753,299 to Lunghofer et al. Indeed, the American Petroleum Institute's (“API's”) description of the proppant qualification process has a section dedicated to the evaluation of roundness and sphericity as measured on the Krumbein scale. The more spherical the proppant, the better it is believed to perform in the proppant pack.
Another property that impacts a proppant's utility is how quickly it settles both in the injection fluid and once it is in the fracture. A proppant that quickly settles may not reach the desired propping location in the fracture, resulting in a low level of proppants in the desired fracture locations, such as high or deep enough in the fracture to maximize the presence of the proppant in the pay zone (i.e., the zone in which oil or natural gas flows back to the well). This can cause reduced efficacy of the fracturing operation. Ideally, a proppant disperses equally throughout all portions of the fracture. Gravity works against this ideal, pulling particles toward the bottom of the fracture. However, proppants with properly engineered densities and shapes may be slow to settle, thereby increasing the functional propped area of the fracture. How quickly a proppant settles is determined in large part by its specific gravity. Engineering the specific gravity of the proppant for various applications is desirable because an optimized specific gravity allows for a proppant to be better placed within the fracture.
Yet another attribute to consider in designing a proppant is its acid-tolerance, as acids are often used in oil and natural gas wells and may undesirably alter the properties of the proppant. For example, hydrofluoric acid is commonly used to treat oil wells, making a proppant's resistance to that acid of high importance.
Still another property to consider for a proppant is its surface texture. A surface texture that enhances, or at least does not inhibit, the conductivity of the oil or gas through the fracture is desirable. Smoother surfaces offer certain advantages over rough surfaces, such as reduced tool wear and a better conductivity, but porous surfaces may still be desirable for some applications where a reduced density may be useful.
All of these properties, some of which can at times conflict with each other, must be weighed in determining the right proppant for a particular situation. Because stimulation of a well through fracturing is by far the most expensive operation over the life of the well, one must also consider the economics. Proppants are typically used in large quantities, making them a large part of the cost.
Attempts have been made to optimize proppants and methods of using them. Suggested materials for proppants include sand, glass beads, ceramic pellets, and portions of walnuts. The preferred material disclosed in previously-mentioned U.S. Pat. No. 3,497,008 is a hard glass, but it also mentions that sintered alumina, steatite, and mullite could be used. Alumina has conventionally been thought to add strength to a proppant, so many early proppants were made of high-alumina materials, such as bauxite. The strength of these high-alumina materials is believed to be due to the mechanical properties of dense ceramic materials therein. See, e.g., U.S. Pat. Nos. 4,068,718 and 4,427,068, both of which disclose proppants made with bauxite. Bauxite ceramics are known to optimize the toughness of a proppant whereas alumina ceramics optimize their hardness. For example, previously-mentioned U.S. Pat. No. 4,427,068 discloses a proppant comprising a clay containing silica that adds a glassy phase to the proppant, thereby weakening the proppant. Furthermore, the silica of that patent is “free” silica, meaning that it is amorphous and not engaged, for example, to the mullite phase. In general, high amounts of silica reduce the strength of the final proppant. In particular, it is believed that sintered proppants containing more than 2% silica by weight will have reduced strength over those with lower silica contents. Other so-called impurities are also believed to reduce the strength of the proppant.
Early high strength proppants were made using tabular alumina which was a relatively expensive component. For this reason, the industry shifted from using tabular alumina to other alumina sources, such as bauxite. By the late 1970's, the development focus in the industry shifted from high strength proppants to intermediate or lower strength, lower density proppants that were easier to transport and use, and were less expensive. Over the next 20 years, the industry focused on commercialization of lower density proppants and they became commonly used. The primary method of reducing the density of proppants is to replace at least a portion of the higher density alumina with lower density silica. According to U.S. Pat. No. 6,753,299, “the original bauxite based proppants of the early 1970's contained >80% alumina (Cooke). Subsequent generations of proppants contained an alumina content of >70% (Fitzgibbons), 40% to 60% (Lunghofer), and later 30% to <40% (Rumpf, Fitzgibbons).” Thus, as to both product development and proppant use, there was a retreat in the industry from proppants manufactured from high-alumina materials such as bauxite.
Numerous production methods have been suggested for making spherical alumina pellets. For example, U.S. Pat. No. 4,427,068 to Fitzgibbon discloses a method of making sintered pellets using a dry pelletizing process. In that process, which is described in more detail below, pellets are forced to rub against each other in a mixer to increase their sphericity. After mixing, the pellets are sintered in a known fashion. Another known method involves preparing an aqueous feed from the desired pellet materials and continuously atomizing the feed into a layer of already partly dried particles made from the same pellet material. Both of these known methods result in particles having surface irregularities and a less than ideal spherical shape. These properties contribute to an uneven distribution of stress that leads to crushing and the generation of fines. They also contribute to having lower void volumes in the pack, as well as a lower conductivity. The surface irregularities can also have an undesirable abrasive effect on the pumping equipment in the well, and have a higher coefficient of friction that can make the removal of the fracturing fluid from the well more difficult and costly. An alternative production process, electrofusion, has been suggested in U.S. Pat. No. 5,964,291 to Bourne et al. However, electrofusion is not the focus of that patent, and no details are provided regarding how the electrofusion is to be accomplished. Indeed, the applicants are not aware of any electrofused proppant products that have been on the market in the past or are on the market today.
Today, as resources become more scarce, the search for oil and gas involves penetration into deeper geological formations, and the recovery of the raw materials becomes increasingly difficult. Therefore, there is a need for proppants that have an excellent conductivity and permeability even under extreme conditions.